1. Field of the Invention
The present invention relates to a semiconductor device which has circuits constituted, over a substrate having an insulating surface, of thin film transistors (hereinafter referred to as TFTs) and its fabricating process. Particularly, the invention relates to an electro-optical device (also called electronic device) represented by a liquid crystal display device or an EL (electroluminescence) display device which is constituted in such a manner that a pixel portion (pixel circuit) and driving circuits (control circuits) disposed in the periphery of the pixel portion are provided on one and the same substrate and an electro-optical appliance (also called an electronic apparatus) presented by an EL (electroluminescence) display device, and an electric appliance (also called electronic apparatus) on which an electro-optical device is mounted.
In this specification, by semiconductor devices, devices in general which function by utilizing the semiconductor characteristics are referred to, and the above-mentioned electro-optical device and an electric appliance on which the electro-optical device is mounted are also covered by the semiconductor devices.
2. Description of the Related Art
The development of semiconductor devices which each comprises a large-area integrated circuit formed of TFTs on a substrate having an insulating surface is being advanced. Known as representative examples of these semiconductor devices are an active matrix type liquid crystal display device, an EL display device, and a contact type image sensor. Particularly, TFTs (hereinafter referred to as polycrystalline silicon TFTs) each constituted in such a manner that a crystalline silicon film (typically, a polycrystalline silicon film) is rendered into an active layer have a high field effect mobility and thus can form various functional circuits.
For example, in an active matrix type liquid crystal display device, a pixel portion which effects image display by every function block and driving circuits such as shift registers, level shifters, buffers and sampling circuits which are based on CMOS circuits are formed on one substrate. Further, in a contact type image sensor, driving circuits such as sample and hold circuits, shift registers, multiplexed circuits for controlling the pixel portion are formed by the use of TFTs.
Since these driving circuits (also known as peripheral driving circuits) do not always have the same operating condition, the characteristics required of the TFTs naturally differ not a little. In the pixel portion, pixel TFTs functioning as switch elements and auxiliary capacitance storage are provided, and a voltage is applied to the liquid crystal to drive it. Here, the liquid crystal needs to be driven by AC, and the system called frame inversion driving is adopted in many cases. Accordingly, for the characteristics required of the TFTs, it was necessary to keep the OFF-current value (the value of the drain current flowing when a TFT is in OFF-operation) sufficiently low.
Further, the buffer, to which a high driving voltage is applied, had to have its withstand voltage enhanced up to such a degree that the buffer would not be broken even if a high voltage was applied thereto. Further, in order to enhance the current driving ability, it was necessary to sufficiently secure the ON-current value (the value of the drain current flowing when the TFT is in ON-operation).
However, there is the problem that the OFF-current value of a polycrystalline silicon TFT is apt to become high. Further, in case of a polycrystalline silicon TFT, there is observed the deterioration phenomenon that its ON-current value falls as in case of a CMOS transistor used in an IC or the like. The main cause therefor lies in the injection of hot carriers; it is considered that the hot carriers generated by the high electric field in the vicinity of the drain cause the deterioration phenomenon.
As a TFT structure for lowering the OFF-current value, the lightly doped drain (LDD) structure is known. This structure is made in such a manner that, between the channel forming region and the source region or the drain region to which an impurity is added at a high concentration, an impurity region having a low concentration is provided. This low concentration impurity region is known as LDD region.
Further, as a structure for preventing the deterioration of the ON-current value due to the injection of hot carriers, there is known the so-called GOLD (Gate-drain Overlapped LDD) structure. In case of this structure, the LDD region is disposed so as to overlap the gate wiring through the gate insulating film, so that this structure is effective for preventing the injection of hot carriers in the vicinity of the drain to enhance the reliability. For example, Mutsuko Hatono, Hajime Akimoto and Takeshi Sakai: IEDM97 TECHNICAL DIGEST, pp. 523-526, 1997, discloses a GOLD structure by the side wall formed of silicon; and it is confirmed that, according to this structure, a very high reliability can be obtained as compared with the TFTs of other structures.
Further, in the pixel portion of an active matrix type liquid crystal display device, a TFT is disposed to each of several ten millions to several hundred millions of pixels, and these TFTs are each provided with a pixel electrode. At the side of the substrate opposed to the pixel electrode through the liquid crystal, an opposite electrode is provided, thus forming a kind of capacitor with the liquid crystal as a dielectric. Then the voltage applied to each of the pixels is controlled by the switching function of the TFT to thereby control the charges to this capacitor, whereby the liquid crystal is driven, and the quantity of transmitted light is controlled, thus displaying an image.
However, the stored capacitance of this capacitor is gradually decreased due to the leakage current caused for causes pertaining to the OFF-current etc., which in turn becomes the cause for varying the quantity of transmitted light to lower the contrast of the image display. Thus, according to the known technique, a capacitor wiring is provided to form in parallel a capacitor (capacitance storage) other than the capacitor constituted with the liquid crystal as its dielectric, whereby the capacitance lost by the capacitor having the liquid crystal as its dielectric was compensated for.